ABSTRACT
In order to explore the crack development mechanism and damage self-repairing capacity of ECC beams reinforced with hybrid bars, the smart aggregate-based active sensing approach were herein adopted to conduct damage monitoring of ECC beams under cyclic loading. A total of six beams, including five engineered cementitious composite (ECC) beams reinforced with different bars and one reinforcement concrete counterpart, were fabricated and tested under cyclic loading. The ultimate failure modes and hysteresis curves were obtained and discussed herein, demonstrating the multiple crack behavior and excellent ductility of ECC material. The damage of the tested beams was monitored by smart aggregate-based (SA) active sensing method, in which two SAs pasted on both beam ends were used as actuator and sensor, respectively. The time domain analysis, wavelet packet-based energy analysis and wavelet packet-based damage index analysis were performed to quantitatively evaluate the crack development. To evaluate the self-repairing capacity of the beams, a self-repairing index defined by the difference of damage index at loading and unloading peak points was proposed. The results in time domain and wavelet packed analysis were in close agreement with the observed crack development, revealing the feasibility of smart aggregate-based active sensing approach in damage detection for ECC beams. Especially, the proposed damage self-repairing index can describe the same structural re-centering phenomena with the test results, showing the proposed index can be used to evaluate the damage self-repairing capacity.
ABSTRACT
Chromium (Cr) pollution in farmlands is a common environmental issue, that can seriously inhibit plant growth, damage plant cells, and even cause plant death. In this study, bok choy (Brassica campestris L. ssp. chinensis Makino (var. communis Tsen et Lee)) was selected as a model plant to investigate the metabolic response to Cr stress at concentrations of 2.0 mg/L and 8.0 mg/L. Metabolites were identified using gas chromatography-mass spectrometry. Principal component analysis and orthogonal projections to latent structure discriminant analysis revealed the notable effect of Cr stress on the metabolites of bok choy. Under Cr stress, 145 metabolites were identified in the bok choy leaves. At 2.0 mg/L Cr stress, 10 and 26 metabolites changed compared to the control after 7 d and 14 d, respectively. At 8.0 mg/L Cr stress, 24 and 24 metabolites changed significantly after 7 and 14 d, respectively. The data showed that metabolism was affected by the Cr stress concentration and exposure time. Specifically, under the Cr stress, the tricarboxylic acid cycle, glutamine synthetase/glutamate synthase cycle, and partial amino acid metabolic pathways were blocked, inhibiting the normal growth and development of bok choy. The change of citric acid content was the most significant, and the accumulation of citric acid indicated the degree of plant Cr toxicity and resistance. These results would facilitate further dissection of the mechanisms of heavy metal accumulation/tolerance in plants and the effective management of such contamination in vegetable crops by genetic manipulation.
